Silica-Catalysed and Highly Stereoselective Convergent and Nonconvergent Rearrangements of Menthone Enol Acetate Epoxides: Easy Access to the Four α-Hydroxymenthone Stereoisomers.

During a program dedicated to the biocatalytic access of structurally useful α-hydroxy ketones, we made the unexpected observation that epoxy enol acetates derived from (+)- and (-)-menthone stereoselectively rearranged on exposure to silica through a diastereoselective process, providing easy access to the complete set of α-hydroxymenthone stereoisomers. The absolute configurations of the four stereoisomers were unambiguously established by IR-VCD and X-ray diffraction, thus clarifying some literature discrepancy. Two of the four stereoisomers could also be obtained through astereoconvergent process, thus avoiding the inherent 50 % yield limitations of such diastereoselective reactions. A solvent-free version allowing an extremely rapid reaction (< 2 h) is also described. Finally, the observed diastereoselection was investigated by DFT calculations. [ABSTRACT FROM AUTHOR]